Free-space quantum key distribution in urban daylight with the SPGD algorithm control of a deformable mirror.

Free-space quantum key distribution (QKD) is important to realize a global-scale quantum communication network. However, performing QKD in daylight against the strong background light noise is a major challenge. Here, we develop the stochastic parallel gradient descent (SPGD) algorithm with a deformable mirror to improve the signal-to-noise ratio (SNR). We then experimentally demonstrate free-space QKD in the presence of urban daylight. The final secure key rate of the QKD is 98-419 bps throughout the majority of the daylight hours. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement